The present invention relates to electric switches, and more particularly to high amperage switches.
Switches and circuit breakers which interrupt electric current are extensively employed in residential, industrial and commercial applications. Switches and circuit breakers suffer from the drawback that the operating speed of the switch is dependent upon their mechanism springs and the condition of their contacts. When the switch is operated using too low a speed, an arc will be formed between the contacts that can ultimately or very suddenly destroy the switch as the energy released by the arc has a greater burning effect the longer the arc is maintained. Arcing causes deterioration of the contacts of the breaker and produces arc gases. Arcing also necessitates switches with greater separation between the contacts in the open position to ensure that the arc does not persist with the contacts in the fully open position. During generation of an arc in a switch, metal particles are scattered from the electrical contact elements, which degrades the electrical contact elements by pitting the surface of the contact elements thereby decreasing the surface area available to make electrical contact.
The prior art has used various methods to extinguish the arc generated under fault conditions. These methods include increasing the length of the arc, decreasing the temperature of the arc, and breaking up the arc in a plurality of other arcs. Prior art devices have also been developed to limit the occurrence of arcing or extinguish it rather than to accommodate the effect of arcing on the switch without degradation of the switch reliability or of its design lifetime. In efforts to limit the occurrence of arcing in some types of switchgear, the switch blades/contacts in some prior art designs are surrounded by an enclosed atmosphere of gas or in a vacuum. In other designs, a resistor in parallel to the switch contacts, is used to limit arcing. In a current limiting circuit breaker, the current limiting contacts are in series with the main contacts of a breaker where a resistor, having a positive temperature coefficient of resistance has its resistance increased due to short circuit current flow thereby limiting the short circuit current buildup. In yet other prior art devices, mechanical means are used to break the arc including inserting an electrical insulating screen or wedge between the contacts during opening of the switch contacts. Notwithstanding the advent of such prior art devices, circuit breakers and switches still have a significant amount of arcing accompanying their operation.
With high amperage switches (above approximately 400 amps), contact pressure is extremely important. With low contact pressure, the possibility of mechanical failure from an overheating condition in the switch increases dramatically as the amperage of the switch increases. Similarly, low contact pressure can cause major problems including mechanical failure of the switch when high fault currents are passed through the switch.
It would therefore be an advantage over the prior art to obtain higher contact pressure between the electrical contacts of the switch without increasing the switch mechanism actuation force which is required to turn the switch on or close the circuit. Increasing the actuation force is undesirable since it makes the switch difficult to operate or turn on, especially when the switch is used in busway or panelboard locations which have limited space and have smaller operating handles to operate the switch.
It would therefore be a further advantage over the prior art to provide a switch with electrical contacts which prevent undesirable contact degradation as the switch is cycled and which can cause the switch not to close completely leading to overheating.
Most prior art high amperage switches have flat contact blades that rely on heavy contact springs for providing the necessary contact pressure between the switch contacts. As the contact pressure increases, it becomes more difficult to turn on the switch, particularly with high amperage blade type switches which have large contact blades. Thus, most high amperage blade type switches are difficult to turn on or off.
Most high amperage blade type switches also are subject to other problems as it is tuned on and off under load. Due to large overload currents, the switch contacts quickly degrade due to pitting from arcing until the friction from the pitted switch contacts is greater than that which the switch mechanism can overcome. Once the friction force is greater than that which the switch mechanism can provide, the switch contacts will not fully engage one another causing the switch to rapidly overheat under electric loading.
As a result of the degradation and the elimination of portions of the electrical contact surface of the switch contact blades from arcing, prior art switches need progressively higher actuation springs in order to obtain /maintain the needed increased pressure between the progressively degraded electrical contacts of the switch as the switch cycled.
It would therefore be an advantage over the prior art to have a switch which permits the use of a high contact spring force without the correspondingly high mechanism actuator force typically needed with a switch which utilizes a high contact spring force.
These and other objects of the invention are achieved by providing an electric switch comprising a housing, a line side strap, a load side strap, a switch blade for establishing electrical continuity by making electrical contact to the line slide strap and to the load side strap and for breaking electrical contact to the line side strap and to the load side strap, the blade having a raised contact surface positioned toward a proximate end, a blade spring for contact with and biasing the switch blade toward the line side strap and the load side strap, and where the switch blade further has a longitudinal edge and a far edge, and a notch is formed into the switch blade from the longitudinal edge toward the far edge, the notch being disposed toward the proximate end of the switch blade, and the raised electrical contact surface of the switch blade is positioned between the proximate end and the notch.